Method of Making Light Emitting Device With Multilayer Silicon-Containing Encapsulant

ABSTRACT

A method of making an LED light emitting device is disclosed. The method includes forming a multilayer encapsulant in contact with an LED by contacting the LED with a first encapsulant that is a silicone gel, silicone gum, silicone fluid, organosiloxane, polysiloxane, polyimide, polyphosphazene, sol-gel composition, or a first photopolymerizable composition, and then contacting the first encapsulant with a second photopolymerizable composition. Each photopolymerizable composition includes a silicon-containing resin and a metal-containing catalyst, the silicon-containing resin comprising silicon-bonded hydrogen and aliphatic unsaturation. Actinic radiation having a wavelength of 700 nm or less is applied to initiate hydrosilylation within the silicon-containing resins.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/747516, filed May 17, 2006, incorporated herein by reference.

FIELD

This disclosure relates to a method of making a light emitting device,and more particularly, to a method of making a light emitting devicehaving a light emitting diode (LED) and a multilayer silicon-containingencapsulant.

BACKGROUND

LED light emitting devices can be manufactured in a variety ofconfigurations, many of which incorporate one or two conductive metalwires connecting a semiconductor or LED die to electrodes in the base ofan LED package. The bonding points of the wires to the electrodes and/orthe LED die are known to be failure points for LED light emittingdevices, and care must be exercised in handling the devices in order toavoid damaging the wire bonds. LED die are typically encapsulated with atransparent organic resin, which serves to both increase the amount oflight extracted from the die as well as to protect the die from physicaldamage.

In general, there is a need for transparent organic resins that can beused as encapsulants for LED light emitting devices. Resins havingrelatively rapid cure mechanisms are desirable in order to acceleratemanufacturing times and reduce overall device cost.

SUMMARY

Disclosed herein is a method of making a light emitting device. In oneaspect, the method comprises: providing a light emitting diode; andforming a multilayer encapsulant in contact with the light emittingdiode, wherein forming the multilayer encapsulant comprises: contactingthe light emitting diode with a first encapsulant comprising a siliconegel, silicone gum, silicone fluid, organosiloxane, polysiloxane,polyimide, polyphosphazene, or sol-gel composition; contacting the firstencapsulant with a photopolymerizable composition comprising asilicon-containing resin and a metal-containing catalyst, thesilicon-containing resin comprising silicon-bonded hydrogen andaliphatic unsaturation; and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thesilicon-containing resin thereby forming a second encapsulant.

In another aspect, the method comprises: providing a light emittingdiode; and forming a multilayer encapsulant in contact with the lightemitting diode, wherein forming the multilayer encapsulant comprises:contacting the light emitting diode with a first photopolymerizablecomposition comprising a first silicon-containing resin and a firstmetal-containing catalyst, the first silicon-containing resin comprisingsilicon-bonded hydrogen and aliphatic unsaturation; and contacting thefirst photopolymerizable composition with a second photopolymerizablecomposition comprising a second silicon-containing resin and a secondmetal-containing catalyst, the second silicon-containing resincomprising silicon-bonded hydrogen and aliphatic unsaturation; andapplying actinic radiation having a wavelength of 700 nm or less toinitiate hydrosilylation within the first and second silicon-containingresins thereby forming first and second encapsulants, respectively.

In yet another aspect, the method comprises: providing a light emittingdiode; and forming a multilayer encapsulant in contact with the lightemitting diode, wherein forming the multilayer encapsulant comprises:contacting the light emitting diode with a first photopolymerizablecomposition comprising a first silicon-containing resin and a firstmetal-containing catalyst, the first silicon-containing resin comprisingsilicon-bonded hydrogen and aliphatic unsaturation; and contacting thefirst photopolymerizable composition with a second photopolymerizablecomposition comprising a second silicon-containing resin and a secondmetal-containing catalyst, the second silicon-containing resincomprising silicon-bonded hydrogen and aliphatic unsaturation; andcontacting the second photopolymerizable composition with a thirdphotopolymerizable composition comprising a third silicon-containingresin and a third metal-containing catalyst, the thirdsilicon-containing resin comprising silicon-bonded hydrogen andaliphatic unsaturation; and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thefirst, second, and third silicon-containing resins thereby formingfirst, second, and third encapsulants, respectively.

The method disclosed herein provides an LED light emitting device withnumerous advantages. With the appropriate selection of materials, theproperties of the first and second encapsulants can be tunedindividually to provide a device with improved photochemical and thermalstability, resistance to physical defects, extended lifetime, andincreased optical efficiency.

These and other aspects of the invention will be apparent from thedetailed description below. In no event, however, should the abovesummary be construed as a limitation on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTIONS OF DRAWINGS

FIG. 1 is a schematic diagram of an LED light emitting device known inthe art.

FIGS. 2 and 3 are schematic diagrams of exemplary LED light emittingdevices as disclosed herein.

DETAILED DESCRIPTION

FIG. 1 shows LED light emitting device 10 in which an LED die isencapsulated with an encapsulant. LED die 11 is mounted on metallizedcontact 12 a disposed on substrate 13 in reflecting cup 14. LED die 11has one electrical contact on its lowermost surface and another on itsuppermost surface, the latter of which is connected to separatemetallized contact 12 b by wire bond 15. A power source (not shown) canbe coupled to the metallized contacts to energize the LED die.Encapsulant 16 encapsulates the LED die.

A known failure mode for LED light emitting devices such as the oneshown in FIG. 1, is breakage of the wire bond as the device undergoesmany cycles of heating and cooling, with temperatures in excess of 130°C. followed by cooling back to ambient temperature. Repeated thermalcycling can subject the wire bond to stress when an encapsulant having ahigh modulus and a high coefficient of thermal expansion is used, andthis stress can ultimately lead to device failure. Epoxies are examplesof such encapsulants. One solution to this problem is to use a lowmodulus, elastomeric, gel-like, or liquid-like silicon encapsulant, oreven very high molecular weight liquids such as silicone gumsencapsulating the chip and wire bond. These materials, however, sufferfrom other problems including a fragile surface prone to damage duringassembly, shipping, or board population and increased dust pick-up onthe surface of the encapsulant.

One way to provide a soft inner layer with a hard outer layer is toprovide a soft gel silicone with a suitably shaped hard outer shell thatcan be formed separately using an injection molding process. However,this external shell adds cost and complexity to the packaging process,for example, by requiring at least one additional pick and placeoperation. Furthermore, most injection moldable plastics have refractiveindices significantly higher than those of the most photostable siliconematerials (the refractive index of polydimethylsiloxane is approximately1.4). As a result, the package lifetime stability must be traded againstoptical efficiency.

FIG. 2 shows exemplary LED light emitting device 20 which includes anLED die encapsulated with a multilayer encapsulant. LED die 21 ismounted on metallized contact 22 a disposed on substrate 23 inreflecting cup 24. LED die 21 has one electrical contact on itslowermost surface and another on its uppermost surface, the latter ofwhich is connected to separate metallized contact 22 b by wire bond 25.A power source (not shown) can be coupled to the metallized contacts toenergize the LED die. A multilayer encapsulant comprising first andsecond encapsulants, 26 and 27, respectively, encapsulates the LED die.

The multilayer encapsulant provides several advantages. The propertiesof the first and second encapsulants can be tuned individually with theappropriate selection of materials. For example, the first encapsulantcan be designed to protect the second encapsulant from high heat andlight generated by the LED die. (LED die temperature can reach over 125°C.) The first encapsulant can also be selected such that it is softerthan the second encapsulant, or it can have lower modulus or viscosityrelative to the second encapsulant. These properties can help reduce theamount of stress on the wire bond(s) during thermal cycling of thedevice such that damage to the wire bond(s) is minimized. The secondencapsulant can be selected such that it provides a hard outer surfacethat resists scratching and other types of physical defects that mightotherwise affect the optical characteristics of light emitted by thedevice. The second encapsulant can also be selected so that italleviates dust pick up issues common to current commercialencapsulants. Another advantage is that the first and secondencapsulants can also be designed to increase the optical efficiency ofthe device, for example, if the refractive index of the firstencapsulant is greater than that of the second.

In one embodiment, the method comprises: providing a light emittingdiode; and forming a multilayer encapsulant in contact with the lightemitting diode, wherein forming the multilayer encapsulant comprises:contacting the light emitting diode with a first encapsulant comprisinga silicone gel, silicone gum, silicone fluid, organosiloxane,polysiloxane, polyimide, polyphosphazene, or sol-gel composition;contacting the first encapsulant with a photopolymerizable compositioncomprising a silicon-containing resin and a metal-containing catalyst,the silicon-containing resin comprising silicon-bonded hydrogen andaliphatic unsaturation; and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thesilicon-containing resin thereby forming a second encapsulant.

Suitable materials for the first encapsulant include those that arethermally stable, photochemically stable, clear and colorless in nature.Herein, thermally stable refers to a material that does not chemicallydegrade upon prolonged exposure to heat, particularly with respect tothe formation of colored or light absorbing degradation products.Herein, photochemically stable refers to a material that does notchemically degrade upon prolonged exposure to actinic radiation,particularly with respect to the formation of colored or light absorbingdegradation products.

The first encapsulant may be a soft material, i.e., softer than thesecond encapsulant. Soft materials include silicone gel or anon-reactive or reactive liquid or gel material that is photo andthermally stable that exerts little to no stress on the wire bond. Thefirst encapsulant may comprise an organosiloxane-containing liquid, gel,elastomer, or a non-elastic solid. Preferred liquid materials arenon-functional silicone fluids and silicone gums, curing silicone fluidsthat build viscosity on irradiation, silicone gums made from lowmolecular weight fluids that cure and chain extend into gum likematerials (i.e. no crosslinking) on irradiation with UV light, andcuring silicone gels. As used herein, fluid refers to a material thatflows readily as compared to a gum which does not. The first encapsulantlayer may also comprise colorless soluble polyimides such as thosedescribed in U.S. Pat. No. 5,750,641, polyphosphazenes, polysiloxanes,and sol gel compositions. In one embodiment, the first encapsulantcomprises a silicone gel, silicone gum, silicone fluid, organosiloxane,polysiloxane, polyimide, polyphosphazene, or sol-gel composition. Forexample, the first encapsulant may comprise an organosiloxane, and theorganosiloxane comprises a trimethylsilyloxy-terminatedpolydimethylsiloxane.

The second encapsulant comprises a silicon-containing encapsulant whichis advantageous because of its thermal and photochemical stability.Silicon-containing encapsulants are known in the art. These compositionstypically comprise organosiloxanes that are cured either byacid-catalyzed condensation reactions between silanol groups bonded tothe organosiloxane components or by metal-catalyzed hydrosilylationreactions between groups incorporating aliphatic unsaturation andsilicon-bonded hydrogen which are bonded to the organosiloxanecomponents. In the first instance, the curing reaction is relativelyslow, sometimes requiring many hours to proceed to completion. In thesecond instance, desirable levels of cure normally require temperaturessignificantly in excess of room temperature. For example, US PatentApplication Publication US 2004/0116640 A1 states that such compositionsare “. . . preferably cured by heating at about 120 to 180° C. for about30 to 180 minutes.”

The second encapsulant is formed from a photopolymerizable compositionas described in U.S. Pat. No. 7,192,795 B1. The photopolymerizablecomposition comprises organosiloxane compositions that are cured bymetal-catalyzed hydrosilylation reactions between groups incorporatingaliphatic unsaturation and silicon-bonded hydrogen, which are bonded tothe organosiloxane components. However, the metal-containing catalystsused herein can be activated by actinic radiation. The advantages ofusing radiation-activated hydrosilylation to cure the compositionsinclude (1) the ability to cure the encapsulating composition withoutsubjecting the LED die, the substrate to which it is attached, or anyother materials present in the package or system, to potentially harmfultemperatures, (2) the ability to formulate one-part encapsulatingcompositions that display long working times (also known as bath life orshelf life), (3) the ability to cure the encapsulating formulation ondemand at the discretion of the user, and (4) the ability to simplifythe formulation process by avoiding the need for two-part formulationsas is typically required for thermally curable hydrosilylationcompositions.

The second encapsulant can be in the form of an elastomer or anon-elastic solid, and is thermally and photochemically stable. Thephotopolymerizable composition comprises a silicon-containing resin thatcan be cured by a rapid cure mechanism (e.g., seconds to less than 30minutes) in order to accelerate manufacturing times and reduce overallcost.

The silicon-containing resin can include monomers, oligomers, polymers,or mixtures thereof. It includes silicon-bonded hydrogen and aliphaticunsaturation, which allows for hydrosilylation (i.e., the addition of asilicon-bonded hydrogen across a carbon-carbon double bond or triplebond). The silicon-bonded hydrogen and the aliphatic unsaturation may ormay not be present in the same molecule. Furthermore, the aliphaticunsaturation may or may not be directly bonded to silicon.

Examples of suitable silicon-containing resins are disclosed, forexample, in U.S. Pat. No. 6,376,569 (Oxman et al.), U.S. Pat. No.4,916,169 (Boardman et al.), U.S. Pat. No. 6,046,250 (Boardman et al.),U.S. Pat. No. 5,145,886 (Oxman et al.), U.S. Pat. No. 6,150,546 (Butts),and in U.S. Pat. Appl. Nos. 2004/0116640 (Miyoshi). A preferredsilicon-containing resin comprises an organosiloxane (i.e., silicones)which includes organopolysiloxanes. Such resins typically include atleast two components, one having silicon-bonded hydrogen and one havingaliphatic unsaturation. However, both silicon-bonded hydrogen andolefinic unsaturation may exist within the same molecule.

In one embodiment, the silicon-containing resin can include a siliconecomponent having at least two sites of aliphatic unsaturation (e.g.,alkenyl or alkynyl groups) bonded to silicon atoms in a molecule and anorganohydrogensilane and/or organohydrogenpolysiloxane component havingat least two hydrogen atoms bonded to silicon atoms in a molecule.Preferably, a silicon-containing resin includes both components, withthe silicone containing aliphatic unsaturation as the base polymer(i.e., the major organosiloxane component in the composition.) Preferredsilicon-containing resins are organopolysiloxanes. Such resins typicallycomprise at least two components, at least one of which containsaliphatic unsaturation and at least one of which contains silicon-bondedhydrogen. Such organopolysiloxanes are known in the art and aredisclosed in such patents as U.S. Pat. No. 3,159,662 (Ashby), U.S. Pat.No. 3,220,972 (Lamoreauz), U.S. Pat. No. 3,410,886 (Joy), U.S. Pat. No.4,609,574 (Keryk), U.S. Pat. No. 5,145,886 (Oxman, et. al), and U.S.Pat. No. 4,916,169 (Boardman et. al). Curable one componentorganopolysiloxane resins are possible if the single resin componentcontains both aliphatic unsaturation and silicon-bonded hydrogen.

Organopolysiloxanes that contain aliphatic unsaturation are preferablylinear, cyclic, or branched organopolysiloxanes comprising units of theformula R¹ _(a)R² _(b)SiO_((4−a−b)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substitutedhydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; R² is a monovalent hydrocarbon group havingaliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1, 2, or3; b is 0, 1, 2, or 3; and the sum a+b is 0, 1, 2, or 3; with theproviso that there is on average at least 1 R² present per molecule.

Organopolysiloxanes that contain aliphatic unsaturation preferably havean average viscosity of at least 5 mPa·s at 25° C.

Examples of suitable R¹ groups are alkyl groups such as methyl, ethyl,n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n-pentyl,iso-pentyl, neo-pentyl, tert-pentyl, cyclopentyl, n-hexyl, cyclohexyl,n-octyl, 2,2,4-trimethylpentyl, n-decyl, n-dodecyl, and n-octadecyl;aromatic groups such as phenyl or naphthyl; alkaryl groups such as4-tolyl; aralkyl groups such as benzyl, 1-phenylethyl, and2-phenylethyl; and substituted alkyl groups such as3,3,3-trifluoro-n-propyl, 1,1,2,2-tetrahydroperfluoro-n-hexyl, and3-chloro-n-propyl. In one embodiment, at least 20 mole percent of the R¹groups are aryl, aralkyl, alkaryl, or combinations thereof. In anotherembodiment, the R¹ groups are methyl, phenyl, or a combination thereof.

Examples of suitable R² groups are alkenyl groups such as vinyl,5-hexenyl, 1-propenyl, allyl, 3-butenyl, 4-pentenyl, 7-octenyl, and9-decenyl; and alkynyl groups such as ethynyl, propargyl and 1-propynyl.In the present invention, groups having aliphatic carbon-carbon multiplebonds include groups having cycloaliphatic carbon-carbon multiple bonds.

Organopolysiloxanes that contain silicon-bonded hydrogen are preferablylinear, cyclic or branched organopolysiloxanes comprising units of theformula R¹ _(a)H_(c)SiO_((4−a−c)/2) wherein: R¹ is as defined above; ais 0, 1, 2, or 3; c is 0, 1, or 2; and the sum of a+c is 0, 1, 2, or 3;with the proviso that there is on average at least 1 silicon-bondedhydrogen atom present per molecule. In one embodiment, at least 90 molepercent of the R¹ groups are methyl. In another embodiment, the R¹groups are methyl, phenyl, or a combination thereof.

Organopolysiloxanes that contain silicon-bonded hydrogen preferably havean average viscosity of at least 5 mPa·s at 25° C.

Organopolysiloxanes that contain both aliphatic unsaturation andsilicon-bonded hydrogen preferably comprise units of both formulae R¹_(a)R² _(b)SiO_((4−a−b)/2) and R¹ _(a)H_(c)SiO_((4−a−c)/2). In theseformulae, R¹, R², a, b, and c are as defined above, with the provisothat there is an average of at least 1 group containing aliphaticunsaturation and 1 silicon-bonded hydrogen atom per molecule.

The molar ratio of silicon-bonded hydrogen atoms to aliphaticunsaturation in the silicon-containing resin (particularly theorganopolysiloxane resin) may range from 0.5 to 10.0 mol/mol, preferablyfrom 0.8 to 4.0 mol/mol, and more preferably from 1.0 to 3.0 mol/mol.

For some embodiments, organopolysiloxane resins described above whereina significant fraction of the R¹ groups are phenyl or other aryl,aralkyl, or alkaryl are preferred, because the incorporation of thesegroups provides materials having higher refractive indices thanmaterials wherein all of the R¹ radicals are, for example, methyl.

The disclosed method involves the use of actinic radiation having awavelength of less than or equal to 700 nanometers (nm). Thus, thedisclosed methods are particularly advantageous to the extent they avoidharmful temperatures. Preferably, the disclosed methods involve theapplication of actinic radiation at a temperature of less than 120° C.,more preferably, at a temperature of less than 60° C., and still morepreferably at a temperature of 25° C. or less.

Actinic radiation used in the disclosed methods includes light of a widerange of wavelengths less than or equal to 700 nm, including visible andUV light, but preferably, the actinic radiation has a wavelength of of600 nm or less, and more preferably from 200 to 600 nm, and even morepreferably, from 250 to 500 nm. Preferably, the actinic radiation has awavelength of at least 200 nm, and more preferably at least 250 nm.

A sufficient amount of actinic radiation is applied to thesilicon-containing resin for a time to form an at least partially curedencapsulant. A partially cured encapsulant means that at least 5 molepercent of the aliphatic unsaturation is consumed in a hydrosilylationreaction. Preferably, a sufficient amount of the actinic radiation isapplied to the silicon-containing resin for a time to form asubstantially cured encapsulant. A substantially cured encapsulant meansthat greater than 60 mole percent of the aliphatic unsaturation presentin the reactant species prior to reaction has been consumed as a resultof the light activated addition reaction of the silicon-bonded hydrogenwith the aliphatic unsaturated species. Preferably, such curing occursin less than 30 minutes, more preferably in less than 10 minutes, andeven more preferably in less than 5 minutes. In certain embodiments,such curing can occur in seconds.

Examples of sources of actinic radiation include those of a very widerange. These include tungsten halogen lamps, xenon arc lamps, mercuryarc lamps, incandescent lamps, fluorescent lamps, lasers, and externalLED illuminators. In certain embodiments, the source of actinicradiation is the LED die, such that applying actinic radiation comprisesactivating the LED die.

The disclosed compositions also include a metal-containing catalystwhich enables the cure of the encapulating material viaradiation-activated hydrosilylation. These catalysts are known in theart and typically include complexes of precious metals such as platinum,rhodium, iridium, cobalt, nickel, and palladium. The preciousmetal-containing catalyst preferably contains platinum. Disclosedcompositions can also include a cocatalyst.

A variety of such catalysts is disclosed, for example, in U.S. Pat. No.6,376,569 (Oxman et al.), U.S. Pat. No. 4,916,169 (Boardman et al.),U.S. Pat. No. 6,046,250 (Boardman et al.), U.S. Pat. No. 5,145,886(Oxman et al.), U.S. Pat. No. 6,150,546 (Butts), U.S. Pat. No. 4,530,879(Drahnak), U.S. Pat. No. 4,510,094 (Drahnak) U.S. Pat. No. 5,496,961(Dauth), U.S. Pat. No. 5,523,436 (Dauth), U.S. Pat. No. 4,670,531(Eckberg), as well as International Publication No. WO 95/025735(Mignani).

Certain preferred platinum-containing catalysts are selected from thegroup consisting of Pt(II) β-diketonate complexes (such as thosedisclosed in U.S. Pat. No. 5,145,886 (Oxman et al.),(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes (such as thosedisclosed in U.S. Pat. No. 4,916,169 (Boardman et al.) and U.S. Pat. No.4,510,094 (Drahnak)), and C₇₋₂₀-aromatic substituted(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes (such as thosedisclosed in U.S. Pat. No. 6,150,546 (Butts).

Such catalysts are used in an amount effective to accelerate thehydrosilylation reaction. Such catalysts are preferably included in theencapsulating material in an amount of at least 1 part, and morepreferably at least 5 parts, per one million parts of the encapsulatingmaterial composition. Such catalysts are preferably included in theencapsulating material in an amount of no greater than 1000 parts ofmetal, and more preferably no greater than 200 parts of metal, per onemillion parts of the encapsulating material composition.

In another embodiment, the method disclosed herein comprises: providinga light emitting diode; and forming a multilayer encapsulant in contactwith the light emitting diode, wherein forming the multilayerencapsulant comprises: contacting the light emitting diode with a firstphotopolymerizable composition comprising a first silicon-containingresin and a first metal-containing catalyst, the firstsilicon-containing resin comprising silicon-bonded hydrogen andaliphatic unsaturation; and contacting the first photopolymerizablecomposition with a second photopolymerizable composition comprising asecond silicon-containing resin and a second metal-containing catalyst,the second silicon-containing resin comprising silicon-bonded hydrogenand aliphatic unsaturation; and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thefirst and second silicon-containing resins thereby forming first andsecond encapsulants, respectively. The first encapsulant may be softerthan the second.

The first and second photopolymerizable compositions may bephotopolymerizable compositions as described above. In this case, theycan be dispensed into the LED package sequentially and then curedsimultaneously to reduce the process time associated with dispensing andcuring a first layer material and then dispensing and curing a secondlayer material. The photocuring silicones would also allow forimprovements in process time for this stepwise curing mechanism as welldue to the rapid cure times achievable with photocuring silicones as wasdescribed in U.S. Pat. No. 7,192,795 B1.

In general, the materials used in the first and secondphotopolymerizable compositions may be selected such that the firstencapsulant is softer than the second. By softer it is meant that thefirst encapsulant is more easily deformed by an external mechanicalforce than the second encapsulant. For example, the first encapsulantmay have a lower Young's modulus or lower Shore Hardness. A softer firstencapsulant may be obtained by an appropriate selection of the first andsecond silicon-containing resins. For example, a softer firstencapsulant may be obtained by having a lower crosslink density than thesecond. This may be achieved by decreasing the number of silicon-bondedhydrogens along the backbone of the first silicon-containing resin,and/or by increasing the molecular weight of the segments betweencrosslinks. A softer first encapsulant may also be obtained by theparticular metal-containing catalyst used in each of thephotopolymerizable compositions. For example, if the samemetal-containing catalyst is used, then a softer first encapsulant maybe obtained by including less of the catalyst in the firstphotopolymerizable composition. If the same silicon-containing resin isuse, then a softer first encapsulant may be obtained by including a lessreactive catalyst in the first photopolymerizable composition.

For the embodiments described herein, the materials used for the firstand second encapsulants may be selected to obtain desired refractiveindices. For example, the first and second encapsulants havesubstantially the same refractive index. For another example, the firstencapsulant may have a refractive index greater than that of the second.This step down in refractive index from light emitting chip to the firstencapsulant to the second encapsulant and finally to air, results inmore efficient light extraction from the package due to minimization oflight loss due Fresnel reflection and absorption. If the encapsulantshave different refractive indices, it is possible for there to be a thingraded index layer at the interface resulting from interdiffusion of thehigh and low index materials. The level of interdiffusion will be afunction of the chemical nature of the materials, the curing mechanism,and rate of cure.

Particularly useful constructions include (1) an LED with a firstencapsulant in contact with the semiconductor die and wire bonds and asecond encapsulant disposed on top of the first encapsulant, wherein thefirst encapsulant has a higher refractive index than the secondencapsulant; (2) an LED with a first silicon-containing encapsulant incontact with the semiconductor die and wire bonds and a secondencapsulant disposed on top of the first encapsulant, wherein the firstencapsulant has a higher refractive index than the second encapsulantand the first encapsulant is a liquid, gel, gum or very soft elastomer;and (3) an LED with a first silicon-containing encapsulant in contactwith the semiconductor die and wire bonds and a second encapsulantdisposed on top of the first encapsulant, wherein the first encapsulanthas a refractive index substantially the same as the second encapsulantand the first encapsulant is a liquid, gel, gum or very soft elastomer.Another embodiment is a two-layer, liquid/gel to solid, two-layerconstruction where the first encapsulant is a non-reactive or reactiveliquid or gel material that is sealed with a reactive solid layer ontop.

For UV light, silicon-containing resins having refractive indices of atleast 1.34 are preferred. For some embodiments, silicon-containingresins having refractive indices of at least 1.50 are preferred.

In another embodiment, a third encapsulant may be used. In this case,the second encapsulant may have a refractive index greater than that ofthe third encapsulant. A particularly useful construction comprises: (4)an LED with a first silicon-containing encapsulant in contact with thesemiconductor die and wire bonds and a second encapsulant disposed ontop of the first encapsulant, wherein the first encapsulant has a higherrefractive index than the second encapsulant and the first encapsulantis a liquid, gel, gum or very soft elastomer, and includes a thirdencapsulant disposed on the second encapsulant wherein the refractiveindex of the second is greater than that of the third encapsulant

The first and/or second encapsulants can comprise one or more additivesselected from the group consisting of nonabsorbing metal oxideparticles, semiconductor particles, phosphors, sensitizers,antioxidants, pigments, photoinitiators, catalyst inhibitors, andcombinations thereof. If used, such additives are used in amounts toproduced the desired effect.

As described above, it may be desirable for the first encapsulant tohave a refractive index greater than that of the second. This may beachieved by including high refractive index nanometer sized particlesthat may or may not be surface modified. If desired, the nanoparticlescan be selected so that they do not introduce color or scattering to theencapsulant.

Nonabsorbing metal oxide and semiconductor particles that aresubstantially transparent over the emission bandwidth of the LED can beused. For example, a 1 mm thick disk of the nonabsorbing metal oxideand/or semiconductor particles mixed with encapsulant may absorb lessthan about 30% of the light incident on the disk. In other cases themixture may absorb less than 10% of the light incident on the disk.Examples of nonabsorbing metal oxide and semiconductor particlesinclude, but are not limited to, Al₂O₃, ZrO₂, TiO₂, V₂O₅, ZnO, SnO₂,ZnS, SiO₂, and mixtures thereof, as well as other sufficientlytransparent non-oxide ceramic materials such as semiconductor materialsincluding such materials as ZnS, CdS, and GaN. The particles can besurface treated to improve dispersibility in the encapsulant. Examplesof such surface treatment chemistries include silanes, siloxanes,carboxylic acids, phosphonic acids, zirconates, titanates, and the like.Techniques for applying such surface treatment chemistries are known.Silica (SiO₂) has a relatively low refractive index but it may be usefulin some applications, for example, as a thin surface treatment forparticles made of higher refractive index materials, to allow for morefacile surface treatment with organosilanes. In this regard, theparticles can include species that have a core of one material on whichis deposited a material of another type.

If used, the nonabsorbing metal oxide and semiconductor particles arepreferably included in the encapsulating material in an amount of nogreater than 85 wt-%, based on the total weight of the encapsulatingmaterial. Preferably, the nonabsorbing metal oxide and semiconductorparticles are included in the encapsulating material in an amount of atleast 10 wt-%, and more preferably in an amount of at least 45 wt-%,based on the total weight of the encapsulating material. Generally theparticles can range in size from 1 nanometer to 1 micron, preferablyfrom 10 nanometers to 300 nanometers, more preferably, from 10nanometers to 100 nanometers. This particle size is an average particlesize, wherein the particle size is the longest dimension of theparticles, which is a diameter for spherical particles. It will beappreciated by those skilled in the art that the volume percent of metaloxide and/or semiconductor particles cannot exceed 74 percent by volumegiven a monomodal distribution of spherical particles.

Phosphors can optionally be included in the encapsulating material toadjust the color emitted from the LED. As described herein, a phosphorconsists of a photoluminescent material. The fluorescent material couldbe inorganic particles, organic particles, or organic molecules or acombination thereof. Suitable inorganic particles include doped garnets(such as YAG:Ce and (Y,Gd)AG:Ce), aluminates (such as Sr₂Al₁₄O₂₅:Eu, andBAM:Eu), silicates (such as SrBaSiO:Eu), sulfides (such as ZnS:Ag,CaS:Eu, and SrGa₂S₄:Eu), oxy-sulfides, oxy-nitrides, phosphates,borates, and tungstates (such as CaWO₄). These materials may be in theform of conventional phosphor powders or nanoparticle phosphor powders.Another class of suitable inorganic particles is the so-called quantumdot phosphors made of semiconductor nanoparticles including Si, Ge, CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, PbS, PbSe, PbTe, InN, InP, InAs, AlN, AlP,AlAs, GaN, GaP, GaAs and combinations thereof. Generally, the surface ofeach quantum dot will be at least partially coated with an organicmolecule to prevent agglomeration and increase compatibility with thebinder. In some cases the semiconductor quantum dot may be made up ofseveral layers of different materials in a core-shell construction.Suitable organic molecules include fluorescent dyes such as those listedin U.S. Pat. No. 6,600,175 (Baretz et al.). Preferred fluorescentmaterials are those that exhibit good durability and stable opticalproperties. The phosphor layer may consist of a blend of different typesof phosphors in a single layer or a series of layers, each containingone or more types of phosphors. The inorganic phosphor particles in thephosphor layer may vary in size (e.g., diameter) and they may besegregated such that the average particle size is not uniform across thecross-section of the siloxane layer in which they are incorporated. Ifused, the phosphor particles are preferably included in theencapsulating material in an amount of no greater than 85 wt-%, and inan amount of at least 1 wt-%, based on the total weight of theencapsulating material. The amount of phosphor used will be adjustedaccording to the thickness of the siloxane layer containing the phosphorand the desired color of the emitted light.

Sensitizers can optionally be included in the encapsulating material toboth increase the overall rate of the curing process (or hydrosilylationreaction) at a given wavelength of initiating radiation and/or shift theoptimum effective wavelength of the initiating radiation to longervalues. Useful sensitizers include, for example, polycyclic aromaticcompounds and aromatic compounds containing a ketone chromaphore (suchas those disclosed in U.S. Pat. No. 4,916,169 (Boardman et al.) and U.S.Pat. No. 6,376,569 (Oxman et al.)). Examples of useful sensitizersinclude, but are not limited to, 2-chlorothioxanthone,9,10-dimethylanthracene, 9,10-dichloroanthracene, and2-ethyl-9,10-dimethylanthracene. If used, such sensitizers arepreferably included in the encapsulating material in an amount of nogreater than 50,000 parts by weight, and more preferably no greater than5000 parts by weight, per one million parts of the composition. If used,such sensitizers are preferably included in the encapsulating materialin an amount of at least 50 parts by weight, and more preferably atleast 100 parts by weight, per one million parts of the composition.

Photoinitiators can optionally be included in the encapsulating materialto increase the overall rate of the curing process (or hydrosilylationreaction). Useful photoinitiators include, for example, monoketals ofα-diketones or α-ketoaldehydes and acyloins and their correspondingethers (such as those disclosed in U.S. Pat. No. 6,376,569 (Oxman etal.)). If used, such photoinitiators are preferably included in theencapsulating material in an amount of no greater than 50,000 parts byweight, and more preferably no greater than 5000 parts by weight, perone million parts of the composition. If used, such photoinitiators arepreferably included in the encapsulating material in an amount of atleast 50 parts by weight, and more preferably at least 100 parts byweight, per one million parts of the composition.

Catalyst inhibitors can optionally be included in the encapsulatingmaterial to further extend the usable shelf life of the composition.Catalyst inhibitors are known in the art and include such materials asacetylenic alcohols (for example, see U.S. Pat. No. 3,989,666 (Niemi)and U.S. Pat. No. 3,445,420 (Kookootsedes et al.)), unsaturatedcarboxylic esters (for example, see U.S. Pat. No. 4,504,645 (Melancon),U.S. Pat. No. 4,256,870 (Eckberg), U.S. Pat. No. 4,347,346 (Eckberg),and U.S. Pat. No. 4,774,111 (Lo)) and certain olefinic siloxanes (forexample, see U.S. Pat. No. 3,933,880 (Bergstrom), U.S. Pat. No.3,989,666 (Niemi), and U.S. Pat. No. 3,989,667 (Lee et al.). If used,such catalyst inhibitors are preferably included in the encapsulatingmaterial in an amount not to exceed the amount of the metal-containingcatalyst on a mole basis.

The light emission for the LED light emitting device can be modified bythe shape and structure of the first encapsulant. FIG. 3 shows exemplaryLED light emitting device 30 which includes an LED die encapsulated witha multilayer encapsulant, wherein the first encapsulant is shaped as asmall lens over the die. LED die 31 is mounted on metallized contact 32a disposed on substrate 33 in reflecting cup 34. LED die 31 has oneelectrical contact on its lowermost surface and another on its uppermostsurface, the latter of which is connected to separate metallized contact32 b by wire bond 35. A power source (not shown) can be coupled to themetallized contacts to energize the LED die. A multilayer encapsulantcomprising first and second encapsulants, 36 and 37, respectively,encapsulates the LED die.

In the embodiment shown in FIG. 3, the small lens over the die can beused to narrow the light distribution emitted from the package. Shapingof the inner encapsulant layer is possible by controlling therheological and wetting properties of the resin and/or controlling thesurface energy of the package material by treatment with organicmonolayers. First encapsulant 36 can have a relatively high refractiveindex.

The multilayer encapsulant described herein is used for light emittingdevices that include an LED. LED in this regard refers to a diode thatemits light, whether visible, ultraviolet, or infrared. It includesencapsulated semiconductor devices marketed as “LEDs”, whether of theconventional or super-radiant variety. Vertical cavity surface emittinglaser diodes are another form of LED. An “LED die” is an LED in its mostbasic form, i.e., in the form of an individual component or chip made bysemiconductor wafer processing procedures. The component or chip caninclude electrical contacts suitable for application of power toenergize the device. The individual layers and other functional elementsof the component or chip are typically formed on the wafer scale, thefinished wafer finally being diced into individual piece parts to yielda multiplicity of LED dies.

The silicon-containing materials described herein are useful with a widevariety of LEDs, including monochrome and phosphor-LEDs (in which blueor UV light is converted to another color via a fluorescent phosphor).They are also useful for encapsulating LEDs packaged in a variety ofconfigurations, including but not limited to LEDs surface mounted inceramic or polymeric packages, which may or may not have a reflectingcup, LEDs mounted on circuit boards, and LEDs mounted on plasticelectronic substrates.

LED emission light can be any light that an LED source can emit and canrange from the UV to the infrared portions of the electromagneticspectrum depending on the composition and structure of the semiconductorlayers. Where the source of the actinic radiation is the LED itself, LEDemission is preferably in the range from 350-500 nm. Thesilicon-containing materials described herein are particularly useful insurface mount and side mount LED packages where the encapsulant is curedin a reflector cup. They are also particularly useful with LED designscontaining a top wire bond (as opposed to flip-chip configurations).Additionally, the silicon containing materials can be useful for surfacemount LEDs where there is no reflector cup and can be useful forencapsulating arrays of surface mounted LEDs attached to a variety ofsubstrates.

The silicon-containing materials described herein are resistant tothermal and photodegradation (resistant to yellowing) and thus areparticularly useful for white light sources (i.e., white light emittingdevices). White light sources that utilize LEDs in their constructioncan have two basic configurations. In one, referred to herein as directemissive LEDs, white light is generated by direct emission of differentcolored LEDs. Examples include a combination of a red LED, a green LED,and a blue LED, and a combination of a blue LED and a yellow LED. In theother basic configuration, referred to herein as LED-excitedphosphor-based light sources (PLEDs), a single LED generates light in anarrow range of wavelengths, which impinges upon and excites a phosphormaterial to produce visible light. The phosphor can comprise a mixtureor combination of distinct phosphor materials, and the light emitted bythe phosphor can include a plurality of narrow emission linesdistributed over the visible wavelength range such that the emittedlight appears substantially white to the unaided human eye.

An example of a PLED is a blue LED illuminating a phosphor that convertsblue to both red and green wavelengths. A portion of the blue excitationlight is not absorbed by the phosphor, and the residual blue excitationlight is combined with the red and green light emitted by the phosphor.Another example of a PLED is an ultraviolet (UV) LED illuminating aphosphor that absorbs and converts UV light to red, green, and bluelight. Organopolysiloxanes where the R¹ groups are small and haveminimal UV absorption, for example methyl, are preferred for UV lightemitting diodes. It will be apparent to one skilled in the art thatcompetitive absorption of the actinic radiation by the phosphor willdecrease absorption by the photoinitiators slowing or even preventingcure if the system is not carefully constructed.

The LED may have an optical element encapsulated in it, as described inU.S. Ser. No. 11/383916 (Attorney docket no. 62129US002) and US2006/0189013 A1, the disclosures of which are incorporated herein byreference for all that they contain.

EXAMPLES Mounting Blue LED Die in a Ceramic Package

Into a Kyocera package (Kyocera America, Inc., Part No. KD-LA2707-A) wasbonded a Cree XT die (Cree Inc., Part No. C460XT290-0119-A) using awater based halide flux (Superior No. 30, Superior Flux & Mfg. Co.). TheLED device was completed by wire bonding (Kulicke and Soffa Industries,Inc. 4524 Digital Series Manual Wire Bonder) the Cree XT die using 1 milgold wire. Prior to encapsulation, each device was tested using a OL 770Spectroradiometer (Optronics Laboratories, Inc.) with a constant currentof 20 mA. The peak emission wavelength of the LED was ˜460 nm.

Formulations

The following formulations were used:

-   F-1: 10.00 g of Dow Corning 6-3495 intermediate +1.00 g of Dow    Corning SYLGARD 184 Curing Agent +100 μL of a solution of 33 mg of    (MeCp)PtMe₃ in 1.00 mL of toluene, prepared in an amber vial.-   F-2: 10.00 g of Dow Corning 6-3495 intermediate +0.25 g of Dow    Corning SYLGARD 184 Curing Agent +100 μL of a solution of 33 mg of    (MeCp)PtMe₃ in 1.00 mL of toluene, prepared in an amber vial.-   F-3: 10.00 g of Dow Corning 6-3495 intermediate +0.10 g of Dow    Corning SYLGARD 184 Curing Agent +100 μL of a solution of 33 mg of    (MeCp)PtMe₃ in 1.00 mL of toluene, prepared in an amber vial.-   F-4: Aldrich 37,840-2 trimethylsilyloxy-terminated    polydimethylsiloxane having a viscosity of approximately 10,000 cps.

Example 1

In order to demonstrate the concept of an encapsulant with a soft innerlayer and hard outer layer, samples were first evaluated for cure inapproximately 1 cm diameter vial caps. Different combinations of theformulations from above were placed into the vial caps in the orderdescribed in Table 1, and the samples were irradiated for 10 minutesunder UVP Blak-Ray Lamp Model XX-15 fitted with two 16 inch Philips TUV15W/G15 T8 bulbs emitting primarily at 254 nm. The resultant plugs ofcured organosiloxane were removed from the vial caps and evaluated forcure and surface tack on both upper and lower surfaces. Observations aredescribed in Table 1.

Control

Into the ceramic package with LED die was placed approximately 2 mg ofF-1. The uncured encapsulant was irradiated for 10 minutes under UVPBlak-Ray Lamp Model XX-15 fitted with two 16 inch Philips TUV 15W/G15 T8bulbs emitting primarily at 254 nm. The cured encapsulant waselastomeric and cured as determined by probing with the tip of atweezer. The efficiency of the LED was measured using an OL 770spectroradiometer and increased from 11.3% before encapsulation to 13.8%after encapsulation.

TABLE 1 Ex. Procedure Observation After Curing 1a 6 drops of F-1 curedand tack free on both upper and lower surfaces 1b 6 drops of F-2 curedbut tacky on both upper and lower surfaces 1c 6 drops of F-3 curedliquid material of slightly higher viscosity than F-3, such that it wasstringy when probed with a spatula 1d 3 drops of F-2, then cured andtack free on upper surface, cured 3 drops of F-1 but tacky on lowersurface 1e 3 drops of F-3, then cured and tack free on upper surface, 3drops of F-1 uncured liquid layer of slightly higher viscosity than F-3on lower surface 1f 3 drops of F-4, then cured and tack free on uppersurface, 3 drops of F-1 uncured liquid layer on lower surface

Example 2

Into the ceramic package with LED die was placed a small drop of F-2over the die and wire bond, followed by F-1 to the top of the cup. Thesample was irradiated for 10 minutes under UVP Blak-Ray Lamp Model XX-15fitted with two 16 inch Philips TUV 15W/G15 T8 bulbs emitting primarilyat 254 nm. The outer layer of the cured encapsulant was a hard elastomerand was cured as determined by probing with the tip of a tweezer. Theefficiency of the LED was measured using an OL 770 spectroradiometer andincreased from 11.8% before encapsulation to 16.0% after encapsulation.

Example 3

Into the ceramic package with LED die was placed a small drop of F-3over the die and wire bond, followed by F-1 to the top of the cup. Thesample was irradiated for 10 minutes under UVP Blak-Ray Lamp Model XX-15fitted with two 16 inch Philips TUV 15W/G15 T8 bulbs emitting primarilyat 254 nm. The outer layer of the cured encapsulant was a hard elastomerand was cured as determined by probing with the tip of a tweezer. Theefficiency of the LED was measured using an OL 770 spectroradiometer andincreased from 11.5% before encapsulation to 15.3% after encapsulation.

Example 4

Into the ceramic package with LED die was placed a small drop of F-4over the die and wire bond, followed by F-1 to the top of the cup. Thesample was irradiated for 10 minutes under UVP Blak-Ray Lamp Model XX-15fitted with two 16 inch Philips TUV 15W/G15 T8 bulbs emitting primarilyat 254 nm. The outer layer of the cured encapsulant was a hard elastomerand was cured as determined by probing with the tip of a tweezer. Theefficiency of the LED was measured using an OL 770 spectroradiometer andincreased from 11.3% before encapsulation to 14.5% after encapsulation.

Example 5

Into the ceramic package with LED die was placed a small piece ofGeneral Electric SE-30 silicone gum over the die and wire bond. The gumwas allowed several hours to flow and level in the bottom of the packageover the die and wire bond, followed by adding F-1 to the top of thecup. The LED package with uncured 2-layer encapsulant was irradiated for10 minutes under UVP Blak-Ray Lamp Model XX-15 fitted with two 16 inchPhilips TUV 15W/G15 T8 bulbs emitting primarily at 254 nm. The outerlayer of the cured encapsulant was a hard elastomer and was cured asdetermined by probing with the tip of a tweezer. The efficiency of theLED was measured using an OL 770 spectroradiometer and increased from10.9% before encapsulation to 13.7% after encapsulation.

Example 6

A first mixture of 10.00 g of the vinyl siloxane base polymerH₂C═CH—SiMe₂O—(SiMePhO)₂₀—SiMe₂—CH═CH₂ (olefin meq wt=1.455 g) and 1.64g of the siloxane crosslinking agent HMe₂SiO—(SiMeHO)₅—(SiMePhO)₅—SiMe₂H(Si—H meq wt=0.159 g), and a second mixture of 10.00 g of the vinylsiloxane base polymer H₂C═CH—SiMe₂O—(SiMe₂O)₁₀₀—SiMe₂—CH═CH₂ (olefin meqwt=3.801 g) and 0.56 g of the siloxane crosslinking agentHMe₂SiO—(SiMeHO)₂₀—(SiMe₂O)₂₀—SiMe₂H (Si—H meq wt=0.142 g) were preparedin two 35 mL amber bottles. A catalyst stock solution was prepared bydissolving 15.6 mg of CpPtMe₃ in 1.00 mL of CH₂Cl₂, and 115 μL and 105μL aliquots of this solution were added, respectively, to each of thetwo siloxane polymer mixtures. A vertical refractive index gradient wasobserved: The refractive index of the first formulation was 1.50 andthat of the second formulation was 1.40, with each mixture equivalent toa C═C/Si—H functionality ratio of 1.5 and contains approximately 100 ppmof Pt. Into a Blue LED package (Stanley) was placed first approximately1 mg of the first formulation followed by approximately 1 mg of thesecond formulation. The LED package was immediately illuminated for 1minute under a UV lamp at 365 nm. Probing the surface of the encapsulantwith the tip of a spatula confirmed that the material was fully curedand elastomeric. The light output of the two-layer encapsulant wasincreased relative to the LED package encapsulated with a singleencapsulant of either the higher of lower refractive index silicone.

Example 7

A first mixture of 10.00 g of the vinyl silsesquioxane base polymer(H₂C═CH—SiO_(3/2))₂-(PhSiO_(3/2))₁₈ (olefin meq wt=1.242 g) and 1.92 gof the siloxane crosslinking agent HMe₂SiO—(SiMeHO)₅—(SiMePhO)₅—SiMe₂H(Si—H meq wt=0.159 g) was prepared in a 35 mL amber vial. A secondmixture of 10.00 g of the vinyl siloxane base polymerH₂C═CH—SiMe₂O—(SiMePhO)₂₀—SiMe₂—CH═CH₂ (olefin meq wt=1.455 g) and 1.64g of the siloxane crosslinking agent HMe₂SiO—(SiMeHO)₅—(SiMePhO)₅—SiMe₂H(Si—H meq wt=0.159 g) and a third mixture of 10.00 g of the vinylsiloxane base polymer H₂C═CH—SiMe₂O—(SiMe₂O)₁₀₀—SiMe₂—CH═CH₂ (olefin meqwt=3.801 g) and 0.56 g of the siloxane crosslinking agentHMe₂SiO—(SiMeHO)₂₀—(SiMe₂O)₂₀—SiMe₂H (Si—H meq wt=0.142 g) were preparedin two 35 mL amber bottles. A catalyst stock solution was prepared bydissolving 15.6 mg of CpPtMe₃ in 1.00 mL of CH₂Cl₂, and 120 μL, 115 μL,and 105 μL aliquots of this solution were added, respectively, to eachof the three siloxane polymer mixtures. A vertical refractive indexgradient was observed: The refractive index of the first formulation was1.55, the second 1.50, and that of the third formulation was 1.40. Eachmixture was equivalent to a C═C/Si—H functionality ratio of 1.5 andcontains approximately 100 ppm of Pt. Into a Blue LED package (Stanley)was placed first approximately 1 mg of the first formulation, followedby approximately 1 mg of the second formulation followed byapproximately 1 mg of the third formulation. The LED was illuminated for1 minute under a UV lamp at 365 nm. Probing the surface of theencapsulant with the tip of a spatula confirms that the material wasfully cured and elastomeric. The light output of the three-layerencapsulant was increased relative to the LED package encapsulated witha single silicone formulation.

Various modifications and alterations to the invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of the invention. It should be understood that the inventionis not intended to be unduly limited by the illustrative embodiments andexamples set forth herein, and that such examples and embodiments arepresented by way of example only with the scope of the inventionintended to be limited only by the claims as set forth herein.

1. A method of making a light emitting device, the method comprising:providing a light emitting diode; and forming a multilayer encapsulantin contact with the light emitting diode, wherein forming the multilayerencapsulant comprises: contacting the light emitting diode with a firstencapsulant comprising a silicone gel, silicone gum, silicone fluid,organosiloxane, polysiloxane, polyimide, polyphosphazene, or sol-gelcomposition; contacting the first encapsulant with a photopolymerizablecomposition comprising a silicon-containing resin and a metal-containingcatalyst, the silicon-containing resin comprising silicon-bondedhydrogen and aliphatic unsaturation; and applying actinic radiationhaving a wavelength of 700 nm or less to initiate hydrosilylation withinthe silicon-containing resin thereby forming a second encapsulant. 2.The method of claim 1, wherein the first encapsulant comprises anorganosiloxane, and the organosiloxane comprises atrimethylsilyloxy-terminated polydimethylsiloxane.
 3. The method ofclaim 1, wherein the silicon-bonded hydrogen and the aliphaticunsaturation are present in the same molecule.
 4. The method of claim 1,wherein the silicon-bonded hydrogen and the aliphatic unsaturation arepresent in different molecules.
 5. The method of claim 1, wherein thephotopolymerizable composition comprises an organosiloxane.
 6. Themethod of claim 1, wherein the photopolymerizable composition comprisesan organosiloxane comprising units of the formula:R¹ _(a)R² _(b)SiO_((4−a−b)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; R² is a monovalent hydrocarbon group havingaliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1, 2, or3; b is 0, 1, 2, or 3; and the sum a+b is 0, 1, 2, or 3; with theproviso that there is on average at least one R² present per molecule.7. The method of claim 6, wherein at least 20 mole percent of the R¹groups are aryl, aralkyl, alkaryl, or combinations thereof.
 8. Themethod of claim 1, wherein the photopolymerizable composition comprisesan organosiloxane comprising units of the formula:R¹ _(a)H_(c)SiO_((4−a−c)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; a is 0, 1, 2, or 3; c is 0, 1, or 2; and the sum ofa+c is 0, 1, 2, or 3; with the proviso that there is on average at leastone silicon-bonded hydrogen present per molecule.
 9. The method of claim8, wherein at least 90 mole percent of the R¹ groups are methyl.
 10. Themethod of claim 1, wherein the silicon-bonded hydrogen and the aliphaticunsaturation are present in a molar ratio of from 0.5 to 10.0.
 11. Themethod of claim 1, wherein the metal-containing catalyst is selectedfrom the group consisting of Pt(II) β-diketonate complexes,(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes, andC₇₋₂₀-aromatic substituted (η⁵-cyclopentadienyl)tri(σ-aliphatic)platinumcomplexes.
 12. The method of claim 1, wherein the photopolymerizablematerial comprises one or more additives selected from the groupconsisting of nonabsorbing metal oxide particles, semiconductorparticles, phosphors, sensitizers, antioxidants, pigments,photoinitiators, catalyst inhibitors, and combinations thereof.
 13. Themethod of claim 1, wherein the first encapsulant has a refractive indexgreater than that of the second.
 14. The method of claim 1, whereinapplying actinic radiation comprises activating the light emittingdiode.
 15. A light emitting device prepared using the method of claim 1.16. A method of making a light emitting device, the method comprising:providing a light emitting diode; and forming a multilayer encapsulantin contact with the light emitting diode, wherein forming the multilayerencapsulant comprises: contacting the light emitting diode with a firstphotopolymerizable composition comprising a first silicon-containingresin and a first metal-containing catalyst, the firstsilicon-containing resin comprising silicon-bonded hydrogen andaliphatic unsaturation; and contacting the first photopolymerizablecomposition with a second photopolymerizable composition comprising asecond silicon-containing resin and a second metal-containing catalyst,the second silicon-containing resin comprising silicon-bonded hydrogenand aliphatic unsaturation; and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thefirst and second silicon-containing resins thereby forming first andsecond encapsulants, respectively.
 17. The method of claim 16, whereinthe silicon-bonded hydrogen and the aliphatic unsaturation of the firstsilicon-containing resin are present in the same molecule.
 18. Themethod of claim 16, wherein the silicon-bonded hydrogen and thealiphatic unsaturation of the first silicon-containing resin are presentin different molecules.
 19. The method of claim 16, wherein the firstphotopolymerizable composition comprises an organosiloxane.
 20. Themethod of claim 16, wherein the first photopolymerizable materialcomprises an organosiloxane comprising units of the formula:R¹ _(a)R² _(b)SiO_((4−a−b)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; R² is a monovalent hydrocarbon group havingaliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1, 2, or3; b is 0, 1, 2, or 3; and the sum a+b is 0, 1, 2, or 3; with theproviso that there is on average at least one R² present per molecule.21. The method of claim 20, wherein the R¹ groups are methyl, phenyl, ora combination thereof.
 22. The method of claim 16, wherein the firstphotopolymerizable material comprises an organosiloxane comprising unitsof the formula:R¹ _(a)H_(c)SiO_((4−a−c)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; a is 0, 1, 2, or 3; c is 0, 1, or 2; and the sum ofa+c is 0, 1, 2, or 3; with the proviso that there is on average at leastone silicon-bonded hydrogen present per molecule.
 23. The method ofclaim 22, wherein the R¹ groups are methyl, phenyl, or a combinationthereof.
 24. The method of claim 16, wherein the silicon-bonded hydrogenand the aliphatic unsaturation of the first silicon-containing resin arepresent in a molar ratio of from 0.5 to 10.0.
 25. The method of claim16, wherein the first metal-containing catalyst is selected from thegroup consisting of Pt(II) β-diketonate complexes,(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes, andC₇₋₂₀-aromatic substituted (η⁵-cyclopentadienyl)tri(σ-aliphatic)platinumcomplexes.
 26. The method of claim 16, wherein the firstphotopolymerizable material comprises one or more additives selectedfrom the group consisting of nonabsorbing metal oxide particles,semiconductor particles, phosphors, sensitizers, antioxidants, pigments,photoinitiators, catalyst inhibitors, and combinations thereof.
 27. Themethod of claim 16, wherein the silicon-bonded hydrogen and thealiphatic unsaturation of the second silicon-containing resin arepresent in the same molecule.
 28. The method of claim 16, wherein thesilicon-bonded hydrogen and the aliphatic unsaturation of the secondsilicon-containing resin are present in different molecules.
 29. Themethod of claim 16, wherein the second photopolymerizable compositioncomprises an organosiloxane.
 30. The method of claim 16, wherein thesecond photopolymerizable material comprises an organosiloxanecomprising units of the formula:R¹ _(a)R² _(b)SiO_((4−a−b)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; R² is a monovalent hydrocarbon group havingaliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1, 2, or3; b is 0, 1, 2, or 3; and the sum a+b is 0, 1, 2, or 3; with theproviso that there is on average at least one R² present per molecule.31. The method of claim 30, wherein the R¹ groups are methyl, phenyl, ora combination thereof.
 32. The method of claim 16, wherein the secondphotopolymerizable material comprises an organosiloxane comprising unitsof the formula:R¹ _(a)H_(c)SiO_((4−a−c)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; a is 0, 1, 2, or 3; c is 0, 1, or 2; and the sum ofa+c is 0, 1, 2, or 3; with the proviso that there is on average at leastone silicon-bonded hydrogen present per molecule.
 33. The method ofclaim 32, wherein the R¹ groups are methyl, phenyl, or a combinationthereof.
 34. The method of claim 16, wherein the silicon-bonded hydrogenand the aliphatic unsaturation of the second silicon-containing resinare present in a molar ratio of from 0.5 to 10.0.
 35. The method ofclaim 16, wherein the second metal-containing catalyst is selected fromthe group consisting of Pt(II) β-diketonate complexes,(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes, andC₇₋₂₀-aromatic substituted (η⁵-cyclopentadienyl)tri(σ-aliphatic)platinumcomplexes.
 36. The method of claim 16, wherein the secondphotopolymerizable material comprises one or more additives selectedfrom the group consisting of nonabsorbing metal oxide particles,semiconductor particles, phosphors, sensitizers, antioxidants, pigments,photoinitiators, catalyst inhibitors, and combinations thereof.
 37. Themethod of claim 16, wherein the first encapsulant has a refractive indexgreater than that of the second.
 38. The method of claim 16, whereinapplying actinic radiation comprises activating the light emittingdiode.
 39. A light emitting device prepared using the method of claim16.
 40. A method of making a light emitting device, the methodcomprising: providing a light emitting diode; and forming a multilayerencapsulant in contact with the light emitting diode, wherein formingthe multilayer encapsulant comprises: contacting the light emittingdiode with a first photopolymerizable composition comprising a firstsilicon-containing resin and a first metal-containing catalyst, thefirst silicon-containing resin comprising silicon-bonded hydrogen andaliphatic unsaturation; and contacting the first photopolymerizablecomposition with a second photopolymerizable composition comprising asecond silicon-containing resin and a second metal-containing catalyst,the second silicon-containing resin comprising silicon-bonded hydrogenand aliphatic unsaturation; and contacting the second photopolymerizablecomposition with a third photopolymerizable composition comprising athird silicon-containing resin and a third metal-containing catalyst,the third silicon-containing resin comprising silicon-bonded hydrogenand aliphatic unsaturation; and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thefirst, second, and third silicon-containing resins thereby formingfirst, second, and third encapsulants, respectively.